LASER AR-40 AR-39 DATING OF PHLOGOPITES F ROM SOUTHERN AFRICAN AND SIBERIAN KIMBERLITES AND THEIR XENOLITHS - CONSTRAINTS ON ERUPTION AGES,MELT DEGASSING AND MANTLE VOLATILE COMPOSITIONS/

Laser-probe Ar-40/Ar-39 analyses of phenocrystal and xenolith-derived phlogopites have been used to evaluate the method for dating kimberlit e emplacement ages and to investigate some of the problems inherent in previous K-Ar and incremental heating Ar-40/Ar-39 analyses of kimberl itic phlogopite. Individual laser Ar-40/Ar-39 spot ages of microphenoc rystal phlogopites for two Siberian kimberlites, Udachnaya and Mir, va ry from an error of the likely emplacement ages to 10% and higher. The mean age of microphenocrysts from another, previously undated pipe, L eningrad, is within the range for other pipes in the same field althou gh the error is high (385+/-22 Ma). Phlogopite from the Obnazhennaya p ipe produced Ar-40/Ar-39 ages substantially older (416+/-36 Ma) than t he poorly constrained but geologically more reasonable age of 135 Ma. The resolution of the laser probe technique reveals substantial hetero geneities in the spatial distribution of Ar-40 within the kimberlite m icrophenocrysts. The minimum ages for individual laser extractions app ear to give the closest approximation to the pipe emplacement age. A m acrocrystic mica from the Udachnaya kimberlite contains considerably m ore ''excess'' Ar-40 in the center of the grain, giving ages >60% high er than the U-Pb perovskite emplacement age of 353-367 Ma. Phlogopites from southern African and Siberian mantle xenoliths in kimberlites sh ow intracrystalline Ar-40 variations indicating incomplete Ar loss dur ing kimberlite ascent. Some Ar loss profiles approximate to those theo retically predicted for volume diffusion while others show profiles in dicating more complex processes. The much lower abundance of radiogeni c Ar-40 in phlogopite microphenocrysts compared to macrocrystal and xe nolith derived micas suggest that this ''excess'' of Ar, causing the a nomalously old K-Ar ages and incremental heating Ar-40/Ar-39 spectra, is probably incorporated within the mantle and progressively degassed during kimberlite ascent.